1. Field of the Invention
The present invention relates to an improvement of an ignition plug cooling device of a vehicle engine in which a first ignition plug and a second ignition plug which make respective electrodes thereof face a combustion chamber are mounted on a cylinder head having the combustion chamber and an intake port and an exhaust port which open in the combustion chamber.
2. Description of Background Art
An engine for a vehicle such as a motorcycle is known for enhancing output performance and low fuel consumption by increasing combustion efficiency. See, for example, Japanese Patent No. 4209440. A vehicle engine is known wherein a pair of first and second ignition plugs which makes respective electrodes thereof face a combustion chamber is mounted on a cylinder head, and the first and second ignition plugs are respectively arranged on an inlet side and an outlet side of an air flow passage which is formed in the cylinder head so as to allow a flow of air to pass therethrough for cooling the first and second ignition plugs.
In the above-mentioned conventional ignition plug cooling device of the vehicle engine, since the pair of ignition plugs is arranged in one air flow passage, the cooling property of the second ignition plug arranged on an outlet side of the air flow passage is inevitably inferior to the cooling property of the first ignition plug arranged on an inlet side of the air flow passage. However, the actual circumstance is that there is not a sufficient space in the cylinder head which allows the formation of two independent air flow passages which individually cool the first and second ignition plugs.